In systems for processing and/or transforming granulated plastic materials, the granular material is transported from a storage container to one or more processing machines, usually consisting of injection or thermoforming presses, by means of a conveying or pneumatic transport system, preferably operating under vacuum.
A vacuum pneumatic transport system comprises at least one loading device of the granular material directly associated with a processing machine, a duct which connects a storage container of the granular plastic material to the above-mentioned loading device and at least one vacuum source. The handling of the granular material from the storage container to the loading device is carried out thanks to the vacuum generated in the duct of the above-mentioned vacuum source, for example a blower or a vacuum compressor.
In the jargon of the art, the loading device is referred to as “feeder” when the vacuum source is integrated into the loading device itself. This solution is adopted in the case of pneumatic transport systems of the “local” type that is systems which associate to each machine a single storage container of granular material, located at a short distance. On the other hand, the loading device is referred to as “receiver”, when the vacuum source is separated therefrom. This latter solution is adopted in the case of pneumatic transport systems of the “centralized” type, that is a system which may allow fluidly connecting at different times the same machine (with the loading device thereof) to different storage containers even located at distances of 100 meters.
Operatively, under the action of the vacuum source, the granular material is drawn from the storage container to later reach—transported by the air drawn along the above-mentioned (referred to as suction or conveying) duct—the collection tank of the loading device, from which—once the loading of the tank is complete—it is put into the processing machine. On the other hand, the transport air is drawn by the loading device to be conveyed towards the vacuum source (in the case of the receiver) or directly ejected outwards (in the case of the feeder). Between the collection tank of the loading device and the vacuum source there is arranged a filter adapted for filtering the air which has just separated from most of the granular material, before it reaches the vacuum source. Generally, the filter is integrated into the loading device, be it a feeder or a receiver.
The control of the transport system is essentially aimed at adjusting the filling of the collection tank of the loading device and, that is, the quantity of granular material to be fed to the processing machine. Generally, the filled level of the collection tank (and therefore the quantity of loaded material) is estimated and thus controlled optimising the filling time of the tank.
In the case of pneumatic transport systems of the “local” type, the filling step coincides with the suction step. In the case of pneumatic transport systems of the centralized type, the filling step comprises a suction or filling step (i.e. withdrawal step of the material directly from the storage container) and a duct cleaning step.
In the case of pneumatic transport systems of the centralized type, wherein the conveying ducts may have lengths of even 100 meters, there is, in fact, an actual risk for the granular material to accumulate along the ducts (for example at the curves), with the formation of clogs which often lead to a shut-down of the system. At each filling cycle it is therefore convenient that the suction duct is cleaned, to avoid not only the formation of clogs of material, but also any contaminations between different materials, if different materials are fed to the same machine going from one cycle to the other. During the cleaning step of the duct, the granular material collected along the duct is sent to the tank and adds to that already conveyed therein during the suction step.
In conventional systems, the operator manually sets up the parameters to optimise the operation of the pneumatic transport system.
The duration of the filling cycle is normally set by the operator such that the sum of the suction and duct cleaning times corresponds to the total time for the optimized filling of the receiver. This is performed so as to avoid that the suction duct of the material is clogged with granules because of an excessive loading or vice versa that the receiver is not fully loaded, thus decreasing the efficiency. If a cleaning step is not provided, the duration of the filling cycle is normally set so that the sum of the suction times corresponds to the total time for the optimised filling of the receiver.
The assessment of the above-mentioned time values, i.e. suction time, and cleaning time if any, takes place by means of a series of empirical tests carried out by the operator during the system start-up, with a consequent waste of both material and time.
Furthermore, it should be noted that if there is a change of material, the operator will have to change again the parameters of the suction and cleaning cycle, by changing times, based on further tests.
Further, if the demand of material by the processing machine associated with the receiver/feeder is lower than that set during the system start-up, for example because of a reduction of hourly production, the operator will have to modify the system parameters again.
It is, therefore, apparent that the optimisation of the filling times of the collection tank is one of the most difficult problems to solve in the pneumatic transport of granular plastic material through conveying ducts.
As disclosed above, the problem is further complicated by the variability of the conditions surrounding the pneumatic transport system. In fact, over time, there may arise, for example, variations of the degree of clogging of the filter, fluidisation, grain size and rheological properties of the material. Accordingly, the parameters set up by the operator for the optimisation of the transport system have to be changed again by trial and error.
In the prior art, optical level devices are already known that are arranged inside the collection tank of the receiver/feeder to detect the reaching of a predetermined filled level. Such optical devices allow to significantly reduce the times ad quantities of materials required in the step of system start-up, but do not allow to adjust the filling times when operating variations occur, for example connected to variations of hourly production. These are, in fact, optical level sensors arranged in a fixed position. Furthermore, such optical devices are significantly affected by the quantity of powder transported, by the colour of the granules, and are also subjected to fouling events. Globally, these are not particularly reliable.
Alternatively to the optical devices, weighing devices have been proposed, consisting for example of loading cells that are arranged at the base of the receiver and determine a filled level of the container based on the detected weight of loaded material. Neither these loading devices are particularly reliable, especially when the quantities to detect are of a few kilograms. In fact, these are very susceptible to vibrations. A pneumatic transport system of material is continuously subjected to vibrations. Suffice it to mention the vibrations induced during the various loading cycles of the granular material. Inside the piping and especially in the receivers/feeders, because of the vacuum and the impact of the granular material on the inner surface of the receivers/feeders, there may occur even very strong vibrations which actually prevent a correct and reliable measurement of the quantity of material loaded in the tank.
Lastly, it should be noted that the operating life cycle of the above-mentioned weighing devices is limited. In fact, over time the vibrations of the transport system tend to easily cause damage to the strain gauges with which such devices are provided.